Variable resistance devices



March 27, 1956 w. D. BUCKMAN 2,740,028

VARIABLE RESISTANCE DEVICES Filed Nov. 19, 1952 CARBON 25 BlfACK W 23 IOINVENTOR WILMER D BUCKMAN By M#% ATTORNEY United States Patent VARIABLERESISTANCE DEVICES Wilmer D. Buckman, Minneapolis, Minn., assignor toMinneapolis-Honeywell Regulator Company, Minneapolis, Minn., acorporation of Delaware Application November 19, 1952, Serial No.321,355 Claims. (Cl. 201-62) This invention pertains to variableresistance sensing devices in general and more specifically to avariable resistance sensing device whose resistance is proportional tothe area of contact between a body of current conductive fluid and aresistive material, the area of contact varying in accordance torelative motion between the fluid and the material.

One object of this invention is to provide an improved variableresistance sensing device wherein a resistive material such as carbon inthe form of blocks, plates, baked coatings, or the like is used as theresistance element and a current conductive fluid is used as a means tocontact the carbon resistance elements.

Another object of this invention is to have in a sensing device aplurality of resistive carbon elements, portions of which are contactedby a globule body of current conductive fluid, said elements beingpositioned so that a tilting of the device about one or more axes willincrease the area of contact between the fluid and one or more elementswhile at the same time decreasing one or more other areas of contact,and said elements and said fluid being associated with circuit meansexternal of said device so that the increase and decrease of said areasre- .sults in a decrease and increase respectively of the re .sistanceimposed by said device in said circuit means.

An additional object of the invention is to provide a switch for adevice such as a torque motor that applies to said torque motor anenergization which is proportional to the amount of tilting of saidswitch away from a normal position, said switch including resistivecarbon and a cunent conductive fluid.

Another object of the invention is to provide a switching device of thecurrent conductive fluid type wherein a novel combination of elementsprovides a sensitive and stable switching action.

A further object of this invention is to provide an improved variableresistance switch suitable for use in interrupting high inductivecircuits.

It is another object of the invention to provide a variable resistanceswitch that minimizes arcing.

Still a further object of the invention is to provide a sensitivevariable resistance device wherein the effort required to produce achange in resistance is very small.

These and other objects of'this invention will become apparent from thespecification together with the draw ings wherein:

Figure 1 is a plan view, partly in section, of the vari able resistancedevice,

Figure 2 is a side sectional view of the device, the view being takenalong section lines 2-2 of Figure 1,

Figure 3 is a sectional view of the device taken along section lines 33of Figure 2, and

Figure 4 is a schematic representation of an electric circuit employingthe variable resistance device.

Referring to Figures 1, 2, and 3 a variable resistance device It) isshown. The variable resistance device is contained within an envelope orhousing 11 which in the preferred embodiment is fabricated out of softglass,

but may be made of various other materials well known by those skilledin the art. An electrical insulation material 12 such as one of theceramics is applied in a thin layer over all of the inside of housing 11except for the top portion of housing 11 as viewed in Figure 2. A pairof carbon block resistive elements 13 and 14, embedded in the ceramic12, lie longitudinally along the bottom of the housing 11. The uppersurfaces of the carbon blocks 13 and 14 are represented as 15 and 16respectively and are not coated with the ceramic 12. A portion of eachof surfaces 15 and 16 are tapered toward the center of the device 10 asat 17 and 18 so as to form, in the preferred embodiment of theinvention, a curved depression along the longitudinal axis of thedevice. The tapering of the blocks 13 and 14 as at 17 and 18 results inthe decreasing of the transverse cross-sectional area of the blocks 13and 14 in the direction toward the center of device 10. Blocks 13 and 14are further tapered along a portion of their sides as at 21 and 22, thisbeing best shown on Figure 1. The purpose for reducing the transversecross-sectional area of the carbon blocks 13 and 14 will become moreapparent as the remainder of the device is described.

An electrode member 23 comprising a long elongated metal conducting baris positioned within housing 11 and spaced away from the pair of carbonblocks 13 and 14 by being secured at its two ends in the ceramic lining12. A plurality of metal damping fins or blades 24 are integral with orsuitably attached to the electrode 23 and are disposed lon itudinallyalong the electrode. The fins 24 are shown to be rectangular havingtheir longest dimension transverse to the longitudinal axis of theelectrode 23. A movable body of conducting fluid 25 herein shown asmercury is contained within the housing 11 and is in engagement withsome of the fins 24 attached to the electrode 23 and to portions of thecarbon blocks 13 and 14. The body of conducting fluid 25 is disposed sothat it may move relative to the carbon blocks 13 and 14 from one end tothe other of the device should the device be tilted or should the devicebe accelerated or decelerated. As the body of conducting fluid 25 ismoved with respect to the carbon blocks 13 and 14, various portions,that is, exposed areas of the blocks are engaged or disengaged by thefluid. The body of conducting fluid is at all times in electricalcontact with some of the fins 24 and thus is at all times in contactwith the electrode member 23. A pair of in leads 31 and 32 are embeddedin carbon blocks 13 and 14 respectively and project through the ceramiclining 12 and the glass housing 11 to the outside of the device andprovide a means of connecting the carbon blocks to an external circuit.Another in lead 33 is electrically secured to the electrode member 23and projects out through the glass housing 11, thus providing a meansfor connecting the electrode 23 to an external electrical circuit.

After all of the above mentioned elements have been assembled within thehousing 11, the housing may be evacuated of air and sealed off as at 34,thus producing a hermetic seal which prevents the device from beingcontaminated. if desired, a gas such as hydrogen may be admitted to thehousing 11, it taking the place of the evacuated air and serving thepurpose of an arc suppressor.

The tapered portions 17 and 18 of the upper surfaces 15 and 16 of carbonblock resistors 13 and 14 respectively are preferably curved orspherical thus forming a concave depression or basin for retaining thecurrent conducting fluid 25. It will be appreciated that the concavedepression or basin in the surface of the blocks provides stability inthe operation of the device in that the current conducting fluid 25 willbe definitely positioned for each position of the housing. However, itis well known by those skilled in the art that a fluid switch having aflat surface is more sensitive to tilting than one having a roundedsurface. It is further known that a fluid switch having a flat surfaceupon which the fluid rests is unstable in the sense .that the fluid isnot definitely positioned for each position of the switch envelope. Theinstant invention employs the concave depression or basin, abovedescribed, in order to enjoy the stability which is inherent with thistype of design. Further, the instant invention ha a higher degree ofsensitivity in the novel selection of components, i. e., carbonresistive elements and a current conductive fluid such as mercury. Ithas been found that the action of the mercury on the carbon elements isanalogous to the use of a slick fill in conventional mercury switches inthat the mercury has no tendency to stick to the carbon elements andthus responds readily to any force which would tend to cause relativemotion between the mercury and the carbon elements.

It will be appreciated that there is electrical continuity of varyingresistance between in lead 33 and in leads 3i and 32 respectively. Thatis, one circuit is established from in lead 33 through electrode 23,fins 24, the conducting fluid 25, the carbon block 13, to in lead 31. Asecond circuit is established from .in lead 33 through electrode Z3,fins 24, the conducting fluid 25, the carbon block 14, to in lead 32.The amount of resistance be tween in leads 33 and 31 and 32 respectivelydepends upon the area of contact between the body of current conductingfluid and the uppermost surfaces of the carbon blocks 13 and 14. ing 11were tilted so that the body of fluid would move to the right as viewedin Figure 2 covering quite a large area of the carbon blocks 13, theresistance between in leads 33 and 31 would become relatively small. Itwill be appreciated that as the area of contact between the body ofconducting fluid 25 and carbon blocl; 13 increases, the area of contactbetween the fluid and carbon block 14 is proportionately decreased.Therefore it follows that as the resistance between terminals 33 and 31decreases, the resistance between in leads 33 and 32 increases. Thereverse condition of resistance change would occur when the housing 11was tilted so that the body of fluid would move to the left as viewed inFigure 2 covering more of carbon block 14- and less of carbon block 13.In general, the amount of resistance between in lead 33 and the in leads31 and 32. will be proportional to the area of contact between the bodyof current conducting fluid and the carbon block resistors 13 and 14.The resistance characteristics of the device It can be altered bysuitable shaping of the carbon block elements 13 and 14- as at thetapered surfaces 17, 18, 21, and 22.

The resistivity of the fluid 25 is very low as compared to theresistivity of the carbon block elements 13 and 14, and therefore, itwill be appreciated that substantially all of the resistance between-thein leads 31 and 32 and the in lead 33 is the resistance in the carbonblock elements.

Figure 4 is a schematic representationof an electric circuit wherein thesubject variable resistance switch is used to control a torque motor 41)having a squirrel cage or eddy current armature (not shown), a pair offield windings 41 and 42., and a phasing condenser 43. The fieldwindings 41 and 42 of the torque motor are connected at 44- while theother ends of the windings are connected by the condenser 43 at junctionpoints 45 and 46. Also connected to junction points 45 and 46 arevariable resistances 1.4 and 13 respectively, each having a wiper 25'which are connected to a common point 33-, which is also at groundpotential. Rheostats 13 and are analogous to the carbon blocks 13 and 14as are wipers 25' analogous to the body of electric current conductingfluid 25. Point 33' is analogous to the in'lead 33 of the varaibleresistance device 10. When the device '10 is in a horizontal positionand assuming that the carbon blocks 13 and 14 are symmetrical with oneanother, itheamount of resistance between the in In other words, if thehousleads 33 and 31 will be equal to the amount of resistance betweenthe in leads 33 and 32. That condition is represented on Figure 4 whenthe two wipers 25' are equidistant from the junction points 45 and 46.Under these conditions and when motor 40 is energized by an alternatingcurrent 50, there would be no torque produced due to the fact that thecurrents flowing through the windings 41 and 42 were equal in magnitudeand in phase relationship. Should the switch be tilted so that the bodyof fluid 25 would move to the right, as viewed in Figure 2, coveringmore of the surface of carbon block 13 and covering less of the surfaceof carbon block 14, the result would be that the wiper 25 of rheostat13' would be repositioned closer to junction point 46 while the wiper 25of the rheostat 14' would be repositioned further away from junctionpoint 45 thus causing the currents flowing through the windings 41 and42 to be di'lferent both in phase relationship and in magnitude, createsa rotating flux which in turn produces a torque of a certain directionand magnitude in the motor 40. if the device it} were tilted in theopposite direction, the fluid 25 would move to the left and the motor 46would have a torque developed therein opposite to the first torque.

The variable resistance device 10 is sensitive to any condition thatwill result in the relative movement between the body of electriccurrent conducting fluid 25 and the carbon resistive elements 13 and 14.The relative motion between the aforementioned fluid and elements may becaused by tilting of the device or rapid acceleration or deceleration ofthe entire housing along the longitudinal axis thereof. The fins 24associated with electrode 23 have in addition to their purpose ofcompleting contact between the fluid and the electrode, the furtherpurpose of damping the movement of the fluid 25 inside the housing 1.The damping of the fluid would be advantageous in many instances wherethe switch was suddenly tilted or accelerated or decelerated andovershooting of the mercury was not desired. Another means of dampingthe motion of the body of fluid 25 (although not shown) would be to usea fluid such as oil around the body of current conducting fluid 25. Thebody of current conducting fluid 25, being heavier than the oil, wouldremain below the oil and have its movement retarded thereby.

The variable resistance device 10 may be adapted for many applicationsincluding sensing devices and switches, one application being where itis used to interrupt high inductive circuits. One advantage here ishaving the current gradually decrease before the break in the circuit.

The variable resistance device 10 may also be made in multiple for oneor more axes of tilting and serving one or more devices such as torquemotors.

The variable resistance device is ideally applicable for use as amodulated motor operator, as opposed to an on-off operator, inapplications where it is desired to modulate the energization to themotor in accordance to the degree of deviation of a sensing device awayfrom a normal or neutral position.

While 1 have shown and described a specific embodiment of thisinvention, further modifications and improvements will occur to thoseskilled in the art. I desire it to be understood, therefore, that thisinvention is not limited to the particular form shown and I intend inthe appended claims to cover all modifications which do not depart fromthespirit and scope of this invention.

I claim as my invention:

1. A device of the class described comprising in combination, a tubularsealed envelope, insulation means lining a portion of the inside of saidenvelope, a pair of resistive carbon members positioned axially alongthe bottom of said envelope, said members being secured to saidinsulation means andhaving portions of their surfaces exposed, aconductin electrode positioned in said envelope substantially paralleltoand spaced fromsaid-plates,eleclast named, and means trical connectionsfor said plates and said electrode, a body of current conducting fluidin said envelope adapted to make contact between said electrode andvarious areas of said exposed surfaces of said members, said areas ofcontact between said fluid and said exposed surfaces varying inaccordance to relative motion between said elements damping saidrelative motion.

2. A device of the class described comprising in combination, a gasfilled tubular sealed envelope, at least two plates of resistivematerial positioned in said envelope, electrode means positioned in saidenvelope, said electrode means being spaced from and adjacent to saidplates, electrical connections for said plates and said electrode means,a body of conducting fluid in said envelope adapted to make contactbetween said electrode means and various portions of said plates, saidportions being determined by relative motion between said body and saidplates, and means including said electrode means damping said relativemotion between said body and said plates.

3. A variable resistance sensing device comprising in combination, ahermetically sealed container, insulation means lining a portion of theinside of said container, first and second carbon blocks separatelysecured to said insulation means along the bottom of said container,electrode means positioned within said container and spaced from andsubstantially parallel to said blocks, a body of current conductivefluid adapted to establish electrical continuity between said electrodeand portions of said blocks, and connection means separately connectingsaid electrode and said blocks to circuit means external of saidcontainer, the resistance between the connection means for saidelectrode and the connection means for said blocks being dependent uponthe area of contact between said body of fluid and said blocks, saidarea varying in accordance to relative motion between said body of fluidand said container.

4. A device of the class described comprising, in combination, arelatively straight tubular envelope, a carbon member positioned in andalong the bottom of said envelope, a conducting electrode spaced fromand substantially parallel to said member, electrical connection meansfor said carbon member and said electrode, and a body of currentconducting fluid in said envelope adapted to make contact between saidenvelope and said carbon member, the area of contact between said fluidand said member varying in accordance to relative motion between saidelements last named.

5. A device of the class described comprising, in combination, a tubularsealed envelope, a carbon member, means including said envelope forpositioning said member in and along the bottom of said envelope, anelectrode spaced from and substantially parallel to said member, and abody of conducting fluid in said envelope adapted to make contactbetween said electrode and said carbon member, the area of contactbetween said fluid and said carbon member varying in accordance torelative motion between said elements last named.

References Cited in the file of this patent UNITED STATES PATENTS1,819,849 Stearns Aug. 18, 1931 2,075,056 Rich Mar. 30, 1937 2,256,833McDonald Sept. 23, 1941

